1. Field of the Invention
The present invention relates to a technique of correcting an electrical offset and so on to be produced in an electric circuit of an optical disc drive.
2. Description of the Related Art
In reading and/or writing data from/on a given optical disc, an optical disc drive irradiates a target track on the optical disc with a laser beam such that a beam spot is formed right on the track, receives, at a photodetector, the beam that has been reflected from the track, and then converts the optical energy received into an electric signal. In order to focus the laser beam just on the data storage layer of the optical disc and make the beam spot follow exactly the target track on the data storage layer, the optical disc drive generates servo signals representing residual errors based on the electric signal. More specifically, the optical disc drive generates a tracking error signal representing the magnitude of shift of the beam spot from the target track or a focus error signal representing the distance of the focal point from the data storage layer, thereby performing a feedback control on the location of the beam spot and the focal point.
A circuit for generating the servo signals (which will be referred to herein as a “servo signal generator”) includes a number of amplifiers. Each of those amplifiers should have an electrical offset, which is hard to eliminate completely. Accordingly, such electrical offsets are superposed one upon the other in the resultant servo signal. If the location of the beam spot or the focal point of the laser beam is controlled with such a servo signal, then the residual error described above is created, thus deteriorating the reading and/or writing performance. For that reason, the conventional optical disc drive takes a measure of correcting such an electrical offset in advance before starting to read and/or write data from/on a given optical disc.
The optical disc drive disclosed in Japanese Laid-Open Publication No. 5-62220 also corrects the electrical offset even while reading and/or writing data. This is because the electrical offset is changeable with the ambient temperature of its associated circuit and needs to be corrected appropriately according to the magnitude of that change.
FIG. 25 shows an exemplary arrangement of functional blocks in a conventional optical disc drive 250. The optical disc drive 250 operates in the following manner. Specifically, first, a laser diode 202 emits a laser beam. Next, the laser beam is transformed by a collimator lens 2003 into a parallel light beam, passed through a beam splitter 2004 and then incident onto an objective lens 2005. In response, the objective lens 2005 converges the parallel light beam, thereby forming a laser beam spot on the data storage layer of a given optical disc 2001. Thereafter, the light beam is reflected back from the data storage layer and then incident onto the objective lens 2005 again, which transforms the reflected light beam into a parallel light beam. Subsequently, the beam splitter 2004 turns the parallel light beam, coming from the objective lens 2005, toward a photodetector 2006. On receiving the parallel light beam, the photodetector 2006 generates and outputs a light quantity signal representing the quantity of the light received. A TE signal generator 2007 generates and outputs a tracking error (TE) signal, representing the magnitude of shift of the location of the laser beam spot from the center of the target track on the optical disc 2001, based on the light quantity signal received.
Meanwhile, in accordance with the light quantity signal, a header detector 2013 detects headers, which are recorded sector by sector as pre-pits on the optical disc 2001, thereby generating a header detection signal. In response to the header detection signal, a detection controller 2008 holds the level of a tracking signal, suspends the emission of the laser beam, and detects the offset of the TE signal. Thereafter, the detection controller 2008 allows the laser diode to emit the laser beam again and stops holding the level of the tracking signal. To perform these control operations, the detection controller 2008 generates various types of control signals. More specifically, in response to the header detection signal, the detection controller 2008 generates and outputs a hold signal to control a control signal generator 2009, a blocking signal to control the laser emission of the laser diode 2002 and a detection control signal to control an offset detector 2010. If the hold signal supplied from the detection controller 2008 instructs that the level of the tracking control signal should be held, then the control signal generator 2009 holds the level of the tracking control signal in accordance with the instruction. If the blocking signal instructs that the laser emission should be stopped, the laser diode 2002 stops emitting the laser beam in accordance with the instruction. And if the detection control signal instructs the offset detector 2010 to detect the offset of the TE signal, the offset detector 2010 follows the instruction.
In accordance with the detection control signal instructing that the offset should be detected, the offset detector 2010 detects the magnitude of electrical offset that has been superposed on the TE signal. Based on the magnitude of offset detected, an offset corrector 2011 generates an offset correction signal representing the magnitude of correction. In accordance with the correction signal generated, the control signal generator 2009 corrects the offset of the TE signal.
The control signal generator 2009 not only corrects the TE signal in accordance with the offset correction signal but also outputs a tracking control signal in accordance with the corrected TE signal such that the beam spot of the laser beam can follow the target track on the optical disc 2001. In response to the tracking control signal, a lens driver 2012 changes the position of the objective lens 2005.
Every time a header is detected during a data reading operation, the optical disc drive 250 corrects the electrical offset of the TE signal. The optical disc drive 250 performs similar operations during a write operation, too.
Thus, even while reading or writing data from/on a given optical disc, the conventional optical disc drive once stops the laser emission and then corrects the electrical offset. That is to say, no data can be read from, or written on, the optical disc while the electrical offset is being corrected. Then, the transfer rate of read data from the optical disc drive to a host computer and the transfer rate of write data from the host computer to the optical disc drive will both decrease so significantly as to make it difficult to always achieve required transfer rates. Particularly when an optical disc drive is expected to read and write a TV program from/on an optical disc simultaneously, the optical disc drive must perform the data reading and writing operations alternately and continuously. For that purpose, such an optical disc drive should achieve much higher transfer rates than conventional ones.
Furthermore, as the storage capacities of optical discs have been significantly increased recently, data must be stored thereon at a higher and higher density. As a result, even higher servo precision is required these days. Thus, the optical disc drive must correct the electrical offset more and more often. In that case, however, the conventional optical disc drive will have to suspend the data reading or writing operation for an even longer amount of time and it will be even harder for the conventional optical disc drive to achieve that high transfer rate expected.
These problems could be solved to a certain extent if the buffer memories of the optical disc drive had an increased storage capacity. However, that is not a beneficial measure to take because the manufacturing cost of the optical disc drive would increase in that situation.